Directional microphones are well known for use in speech systems to minimise the effects of ambient noise and reverberation. It is also known to use multiple microphones when there is more than one talker, where the microphones are either placed near to the source or more centrally as an array. Moreover, systems are also known for selecting which microphone or combination to use in high noise or reverberant environments. In teleconferencing applications, it is known to use arrays of directional microphones associated with an automatic mixer. The limitation of these systems is that they are either characterised by a fairly modest directionality or they are of costly construction.
Microphone arrays are generally designed as free-field devices and in some instances are embedded within a structure. The limitation of prior art microphone arrays is that the inter-microphone spacing is restricted to half of the shortest wavelength (highest frequency) of interest. This means that for an increase in frequency range, the array must be made smaller (thereby losing low frequency directivity) or alternatively a microphones must be added to the array (thereby increasing cost). Another problem with prior art microphone arrays is that the beamwidth decreases with increasing frequency and sidelobes become more problematic. This results in significant off axis “coloration” of the signals. As it is impossible to predict when a talker will speak, there is necessarily a period time during which the talker will be off axis with consequential “coloration” degraded performance.
The following references illustrate the known state of the art:    [1] Michael Brandstein, Darren. Ward, “Microphone arrays”, Springer, 2001.    [2] Gary Elko, “A steerable and variable first-order differential microphone array”, U.S. Pat. No. 6,041,127, Mar. 21, 2000.    [3] Michael Stinson, James Ryan, “Microphone array diffracting structure”, Canadian Patent Application 2,292,357.    [4] Jens Meyer, “Beamforming for a circular microphone array mounted on spherically shaped objects”, Journal of the Acoustical Society of America 109 (1), January 2001, pp. 185-193.    [5] Marc Anciant, “Modélisation du champ acoustique incident au décollage de la fusée Ariane”, July 1996, Ph.D. Thesis, Université de Technologie de Compiègne, France.    (7) S. Dedieu, P. Moquin, “Broadband Constant directivity beamforming for non linear and non axi-symmetric arrays”, UK Patent Application No. 0229059.1, published as EP1429581.    (8) S. Dedieu, P.Moquin, “Method for extending the frequency range of a beamformer without spatial aliasing”, UK Patent Application No. 0229267.0 published as EP1432280.    [9] Morse and Ingard, “Theoretical Acoustics”, Princeton University Press, 1968.
Brandstein and Ward [1] provide a good overview of the state of the art in free-field arrays. Most of the work in arrays has been done in free field, where the size of the array is necessarily governed by the frequency span of interest.
The use of an obstacle in a microphone array is discussed in Elko [2]. Specifically, Elko uses a small sphere with microphone dipoles in order to increase wave-travelling time from one microphone to another and thus achieve better performance in terms of directivity. A sphere is used since it permits analytical expressions of the pressure field generated by the source and diffracted by the obstacle. The computation of the pressure at various points on the sphere allows the computation of each of the microphone signal weights. The spacing limit is given as 2λ/π (approx. 0.64λ) where λ is the shortest wavelength of interest.
M. Stinson and J. Ryan [3] extend the principle of microphone arrays embedded in obstacles to more complex shapes using a super-directive approach and a Boundary Element method to compute the pressure field diffracted by the obstacle. Stinson and Ryan have proven that using an obstacle provides correct directivity in the low frequency domain, when generally other authors use microphone arrays of large size.
The benefit of an obstacle for a microphone array in terms of directivity and localisation of the source or multiple sources is also described in the literature by Jens Meyer [4] and by Marc Anciant [5]. Jens Meyer demonstrates the benefit of adding a sphere on a microphone array compared to a free-field array in terms of broadband performance and noise rejection. Anciant describes the “shadow” area for a 3D-microphone array around a mock-up of the Ariane IV rocket in detecting and characterising the engine noise sources at take-off.
With the exception of Elko [2] (who sets the spacing limit at 2λ/π), the prior art explicitly or implicitly concedes the requirement for a high frequency performance limit defined by an inter-element spacing of λ/2 to avoid grating lobes in free-field.
In the state of the art it has been recognised that the directionality of a microphone can be increased by the use of a structure attached to the microphone. In U.S. Pat. No. 4,115,659, Spanel attaches an exponential re-entrant horn to a microphone in order to reduce the “echo” effect of a handsfree telephone conversation. However, the horn is bulky, can only be aimed in one direction and is therefore not very practical. In U.S. Pat. No. 5,748,757, Kubli and West disclose an image-derived microphone with a collapsible structure. This structure does enhance the directivity but, as with Spanel, only in one direction. Another disadvantage is that the structure extends out from the surface of the device. In U.S. Pat. No. 6,148,089, Akino discloses a first order gradient (i.e. cardioid microphone) mounted in a cavity. The mounting structure allows the cardioid microphone to retain its original directionality. Again this is intended for a portable computer so only one direction is provided. Rühl in U.S. Pat. No. 6,305,732 discloses a directional microphone that is integrated into the dashboard of a car and, as with the prior art discussed above, for only one direction. Moreover, all of the prior art systems discussed above use directional microphones that are more expensive than omni-directional microphones.
In accordance with Applicant's own prior inventions [7 and 8] a microphone array is provided with improved directivity having a reasonably constant beampattem over a frequency range that extends beyond the traditional limitation of the inter-sensor spacing of half a wavelength.
It is an object of the present invention to further improve the method used in [8] by optimising the physical characteristics of the obstacle in which the microphones are embedded. This invention addresses the microphone array restrictions discussed above, as well as those of directional microphones that provide only one direction. The combination of an enclosure with optimised physical characteristics into which simple omnidirectional microphones are embedded, provides a beamformer of superior performance as compared to the known prior art.